Complex modulation in which both amplitude modulation and phase/frequency modulation are effected to increase spectral efficiency is widely used in a variety of communications applications. In a variety of communications applications, high power amplifiers are necessary to assure proper signal power is delivered. High power amplifiers of the Class AB type in particular unfortunately will distort modulated RF signals, particularly those signals which have complex modulation schemes, resulting in intermodulation distortion appearing at the output of the amplifier. These distortion products result in undesired frequency components, and often frequency components that will violate strict FCC bandwidth requirements. A variety of techniques have been employed to reduce intermodulation distortion. These include feedback techniques, feedforward techniques and predistortion. The ultimate goal in each of these attempts to reduce intermodulation distortion is to provide a more linear response from the amplifier. Although feedforward linearization often yields significant signal improvement, and works well with broadband signals, it requires an extra power amplifier, known as an error amplifier, and therefore greatly increases the cost and complexity of the system. Feedback requires a feedback delay, often limiting the usefulness to lower frequency applications.
Predistortion is an alternative to feedforward, and almost always consumes less power. Predistortion and feedforward are often used together if the performance requirements are extreme. This patent addresses only the predistortion aspect.
Predistorters can be of two varieties: 1) RF predistorters, in which the RF is applied to an element or group of elements which operate at the RF frequency, such as an expanding amplifier or nonlinear RF device, and 2) Envelope predistorters, in which the incoming RF is amplitude detected and the level is used to modulate gain and phase adjusters which precede the main power amplifier. In this type, the nonlinear curve shaping networks (analog or digital) operate on the envelope (baseband) signal, at a much lower frequency than the RF. This disclosure of the invention of the present discussed in the "Detailed Description of the Invention" below addresses the second variety.
To utilize predistortion, an input signal to an amplifier must be intentionally distorted in an inverse manner such that when it is applied to the amplifier, the output from the amplifier is linear with respect to the input. The gain (AM to AM modulation) and phase (AM to PM modulation) characteristics of a predistorted signal should be mirror images of the amplifier response to effect the necessary changes in the output signal to linearize the output signal. Predistortion techniques have been used for some time to linearize power amplifiers.
In the present art, there is a time delay issue which limits the speed of operation. For any digital predistorter which uses the envelope amplitude as an index to a correction table, the processes of detection, A/D conversion, table look up, D/A, and AM/PM modulation have to be completed in 5% or less of the data period of the modulation for the predistortion to have a significant cancelling effect. As in the case of adding two vectors roughly 180 degrees apart, small angle errors result in a significant residual. In a system with an eye pattern having data bit transitions at 1 MHz, the delay through the entire predistorter path from the RF input to the RF output should be no more than 50 nS (In most cases the envelope is bandlimited and has very gradual transitions in the time domain. The envelope is continually changing). This places significant speed requirements on the circuit elements, allowing only a 10 nS average budget for each of the elements in this example.
This application addresses the speed problem with a look ahead circuit. Based on the very gradual and well defined envelope transitions, the look ahead circuit provides envelope information that leads the detected envelope by 15% or more With this incorporation, the A/D, look up table, and D/A processes have 20% (in this example) of the data period, rather than 5%, to process the amplitude.
Envelope predistorters using analog circuits and non-linear envelope circuits, while in many applications are faster than the digital approaches because translation in and out of the digital domain is not required, may still benefit from a look ahead approach. These analog circuits and non-linear devices, while useful, are difficult to adjust and do not have the advantages of digital circuits such as arbitrary curve generation and automatic calibration. Because of the extra operations, digital predistortion has been limited to baseband operations in many cases.
Baseband approaches have also been used which have predistortion back in the radio, before modulation up to RF. These techniques have the advantage of digital look ahead, because the modulation can be delayed to meet up with the delayed predistortion. However, these techniques require an intricate information connection between the radio and the power amplifier. Predistortion using only the RF signal without access to the premodulated signal, has the advantage that the amplifier can be a stand alone unit without any input other than the RF. That is, the radio/modulator and the amplifier do not need to be matched, calibrated, or purchased together.
Accordingly, what is needed is a technique to effect analog or digital predistortion using the RF signal alone with a look ahead circuit.